1. Field of the Invention
The present invention relates to foam formulations and manufacturing methods and, in particular, to high performance microcellular foam, methods of manufacture and applications thereof.
2. Description of the Prior Art
Solid cast polyurethane systems generally result from the reaction of an isocyanate, a short chain glycol (or amine), and a long chain polyol. To achieve optimal properties, the number of reactive isocyanate and hydroxyl (amine) groups for a given formulation should be nearly equal. The type of isocyanate, glycol, and polyol can be altered to achieve desired properties. In addition, the hardness or modulus of the resulting polyurethane product can be adjusted through the ratio of long chain to short chain polyols in the system.
Polyurethane systems can be classified according to when and how the components are brought together. xe2x80x9cOne-shotxe2x80x9d systems are formed from mixing the individual components all at once. Quasi-prepolymer systems are those in which a portion of the long chain polyol component is pre-reacted with the isocyanate to form an isocyanate-terminated prepolymer. To form the final product, this prepolymer (typically with an isocyanate content in the 15-25% range) is then reacted with the short chain glycol and the remainder of the long chain polyol component. Full prepolymer systems are prepared by pre-reacting the entire long chain polyol component with the isocyanate. The resulting prepolymer (with an isocyanate content typically less than 12% NCO) is then reacted (or cured) wit h the short chain polyol or amine to complete the reaction. Due to the high degree of reaction control, full prepolymer-based systems generally exhibit the best overall physical and dynamic properties of any polyurethane elastomer.
Microcellular polyurethane foams suitable for use in industrial applications are not very common. The most common use is in parts known as xe2x80x9cjounce bumpersxe2x80x9d which act as damping components in automotive strut suspension systems. The chemical system is based on a polyester-based urethane prepolymer such as the one described in the present invention. However, the isocyanate component most often utilized in the jounce bumpers is 1,5-naphthalene diisocyanate (NDI), as contrasted with methylene diphenyl diisocyanate (MDI)-based systems. MDI-based microcellular systems can be used in other applications including shoe soles, acoustical and isolation damping, engine and tool mounts, and seals and suspension systems. Many of these systems, such as those used in the shoe sole applications, utilize xe2x80x9cquasi-prepolymerxe2x80x9d systems with isocyanate contents in the 15-30% range.
Problems discussed herein relate to deficiencies in solid polyurethane systems for achieving low durometers and proper load profiles. Improvement in foam properties of the current invention over typical microcellular products is achieved through the use of a prepolymer approach versus a quasi-prepolymer approach. The use of microcellular polyurethane foams in mechanical and industrial applications has been plagued with problems. One such problem has to do with the difficulty of achieving low durometer, low modulus solid polyurethane cast elastomers with properties capable of competing with soft ( less than xcx9c70 Shore A) rubbers. The current solution for this problem consists of adding plasticizers to polyurethane formulations to soften solid elastomers to the desired hardness. The problem with this approach is that, while proper plasticization does reduce hardness and modulus effectively, other physical, mechanical, and dynamic properties are negatively affected, particularly cut/tear resistance and overall toughness.
Another typical problem encountered with prior art solid polyurethanes is the inability to achieve the compressive load profiles required for certain roller/wheel conveying systems, such as bowling ball lift wheels/tracks and corrugated zero crush rolls. The current solution for this problem is to obtain proper compressive loads with solid elastomer materials, in which complex design elements are utilized. The problem with this approach is that the design elements lead to stress concentrations and fatigue points in the material, thereby reducing product life. Further, from the processing standpoint, sophisticated designs lead to high tooling and engineering costs, which can lead to inefficient production.
The full prepolymer approach generally imparts improved properties over traditional quasi-prepolymer-based systems. Currently, proper force levels are achieved with solid elastomers through part design. The present invention allows the method of achieving desired load compression profiles using a compressible micro cellular material, thereby allowing a solid cross-section design to improve part performance and reliability. These material and processing modifications result in design simplification, improved part performance and reliability, and improved processing efficiency. Examples will be presented for specific applications, which demonstrate advances in the art.
High performance microcellular polyurethane foam has been developed which is suitable for demanding applications requiring high toughness and excellent dynamic characteristics. The current invention includes full prepolymer systems with isocyanate contents in the 3-12% range. In addition, the high compressibility/extensibility of the cellular material provides a route for producing a low modulus material with physical properties typically not attainable in solid cast polyurethane systems. Such characteristics make the material suitable for numerous applications normally considered outside the realm of conventional solid systems. The unique property profile also allows part design modifications. The present invention includes a method of manufacturing a microcellular polyurethane foam comprising the steps of prepare a curative component by mixing a polyol, water, a foam surfactant, and a catalyst component comprising a standard solid cast polyurethane catalyst and a delayed-action tin catalyst; and mixing the curative component with an isocyanate- terminated prepolymer.
In addition, the techniques used to process the chemical system are different from standard solid cast polyurethane practice. Using particularly controlled water addition, foam surfactant addition, a modified catalyst system, adding a delayed action tin catalyst, and a reduced NCO/OH ratio, the process described herein creates a new high performance microcellular foam. This foam has special application for industrial parts that require a flexible, tough, highly compressible polyurethane material. For example, the present invention is particularly successful in the bowling equipment and the cardboard manufacturing industries.